1. Field of the Invention
The present invention relates to an organic light-emitting diode comprising at least one cyclic phosphazene compound, a light-emitting layer formed from at least one matrix material and at least one emitter material, wherein the at least one matrix material comprises at least one cyclic phosphazene compound, to the use of cyclic phosphazene compounds in organic light-emitting diodes, and to a device selected from the group consisting of stationary visual display units, mobile visual display units and illumination units comprising at least one inventive organic light-emitting diode, and to selected cyclic phosphazene compounds and processes for preparing them.
2. Description of the Background
Organic light-emitting diodes (OLEDs) exploit the propensity of materials to emit light when they are excited by electrical current. OLEDs are of particular interest as an alternative to cathode ray tubes and to liquid-crystal displays for producing flat visual display units. Owing to the very compact design and the intrinsically low power consumption, devices comprising OLEDs are suitable especially for mobile applications, for example for applications in cellphones, laptops, etc., and for illumination.
The basic principles of the way in which OLEDs work and suitable structures (layers) of OLEDs are specified, for example, in WO 2005/113704 and the literature cited therein.
The light-emitting materials (emitters) used may, as well as fluorescent materials (fluorescence emitters), be phosphorescent materials (phosphorescence emitters). The phosphorescence emitters are typically organometallic complexes which, in contrast to the fluorescence emitters which exhibit singlet emission, exhibit triplet emission (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). For quantum-mechanical reasons, when the phosphorescence emitters are used, up to four times the quantum efficiency, energy efficiency and power efficiency is possible.
Of particular interest are organic light-emitting diodes with long operative lifetime, good efficiency, high stability to thermal stress and a low use voltage and operating voltage, and which especially emit light in the blue region of the electromagnetic spectrum.
In order to implement the aforementioned properties in practice, it is not just necessary to provide suitable emitter materials, but the other components of the OLED (complementary materials) must also be matched to one another in suitable device compositions. Such device compositions may comprise, for example, specific matrix materials in which the actual light emitter is present in distributed form. In addition, the compositions may comprise blocker materials, in which case hole blockers, exciton blockers and/or electron blockers may be present in the device compositions. Additionally or alternatively, the device compositions may additionally comprise hole injection materials and/or electron injection materials and/or charge transport materials such as hole conductor materials and/or electron conductor materials. The selection of the aforementioned materials which are used in combination with the actual light emitter has a significant influence on properties including the efficiency and the lifetime of the OLEDs.
The prior art proposes numerous different materials for use in the different layers of OLEDs.
DE 103 30 761 A1 relates to mixtures of organic semiconductors and matrix materials capable of emission, to the use thereof and to electronic components which comprise them. The mixtures according to DE 103 30 761 are formed from at least two substances, one of which serves as a matrix material and the other is an emission material capable of emission. The matrix material is a compound which comprises at least one structural unit of the L=X and/or M=X form, where the X radical has at least one non-bonding electron pair, the L radical is P, As, Sb or Bi, the M radical is S, Se, Te, and can if appropriate also form glasslike layers. DE 103 30 761 A1 specifies numerous different suitable matrix materials which are covered by one of the aforementioned formulae. The matrix material may, among other materials, be a cyclic phosphazene, where the cyclic phosphazene may have from 4 to 14 ring atoms and may be substituted by numerous different substituents on the phosphorus. Specific examples of phosphazenes suitable as matrix materials are not specified in DE 103 30 761 A1, since DE 103 30 761 A1 preferentially specifies specific spiro compounds (not phosphazenes) as matrix materials.
JP 08-283416 specifies cyclic phosphazene compounds and organic thin-film elements which comprise them. These are specific cyclic phosphazenes which have 6 or 8 ring members, and in which the phosphorus atom is substituted by phenoxy groups or N,N′-diphenyl-N-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamino]-3-phenyloxy groups, where at least two of the radicals on the phosphorus are phenoxy groups. According to JP 08-283416, the specific phosphazenes are notable in that they have a high thermal stability and a high amorphous stability, and are free of chlorine residues and other impurities. The specific cyclic phosphazenes are, according to JP 08-283416 A, used in hole transport and hole injection layers.
JP 2006-278549 A relates to an organic light-emitting diode which has a layer which comprises a (cyclo)phosphazene compound. Suitable (cyclo)phosphazene compounds specified in JP 2006-278549 A are (cyclo)phosphazene compounds which have from 4 to 8 ring members. Suitable substituents on the phosphorus are —OR1 and —OR2 groups joined via oxygen, a 6-membered phosphazene with —O—C6H4—CF3 and/or —O—C6H4—F substituents being specified explicitly.
V. Vicente et al. (New. J. Chem., 2004, 28, 418-424) disclose the syntheses and structures of cyclotriphosphazenes with heterocyclic substituents bonded via carbon atoms. The heterocyclic substituents used are 2-thienyl, 3-thienyl and 3,3′-bithienyl-2,2′-ylene groups. On the basis of the geometric properties of the cyclotriphosphazenes specified, it is suspected that they can form stacked structures with possibly interesting electronic properties through coordination of the free heteroatoms to transition metals. No specific possible uses of the cyclotriphosphazenes are mentioned in V. Vicente et al.
V. Vicente et al. Magn. Res. Chem. 2003, 41:183-192 disclose NMR studies of cyclotriphosphazenes. Among others, structures of cyclotriphosphazenes with heterocyclic substituents bonded via carbon atoms are studied by NMR spectroscopy. In addition to the cyclotriphosphazenes substituted by 2-thienyl, 3-thienyl and 3,3′-bithienyl-2,2′-ylene groups disclosed in the aforementioned document, cyclotriphosphazenes which bear 2-pyridyl groups are disclosed, as are cyclotriphosphazenes which bear spiro groups. The NMR studies are intended to serve to elucidate electronic properties of the cyclotriphosphazenes studied. No specific possible uses of the cyclotriphosphazenes are mentioned in V. Vicente et al.
C. Combes-Chamalet et al. J. Chem. Soc., Perkin Trans. 2, 1997 15-18 relates to the synthesis and structure of spiro-cyclotriphosphazenes. These might—like cyclotriphosphazenes—be of interest for the formation of inclusion adducts with water or organic solvents. No specific possible uses of the spiro-cyclotriphosphazenes are mentioned in C. Combes-Chamalet et al.
It is an object of the present invention, with respect to the prior art cited above, to provide materials suitable for use in OLEDs, especially for use as matrix materials in a light-emitting layer, and/or for use as hole blocker materials, said light-emitting layer preferably comprising at least one blue emitter. More particularly, complementary materials for OLEDs (preferably matrix materials, charge transport materials, blocker materials, charge injection materials) shall thus be provided, said materials having a high triplet energy, in order that light emission of the blue emitter used in the OLEDs is ensured. Furthermore, the materials shall be suitable for providing OLEDs which ensure good efficiencies, good operative lifetimes and a high stability to thermal stress, and a low use voltage and operating voltage of the OLEDs.